There’s an old but still active thread on SEOchat forums discussing most useful link building sources.

Here are a few ideas described there (please add your own in the comments!)

Develop Your Own Site

• Create great content: “in the Internet world, superior content creates a lot of luck“. “Build it and they will come – this is most important. From experience, if your content is best in the industry (you know what they say about opinions)“

Build Links Using Social Media

• Follow some niche influencers on Twitter (you may want a separate Twitter list for them) and watch what type of content then like. Then adapt your content strategy accordingly and tweet to them your articles, research, etc

Blog

• Create a blog that has regular contributors, experts per se who will promote their own content – best of all the content in on you site. You are getting free content and free promotion!
• Create crowd-sourced interviews, asking the group of niche influencers one and the same question. Turn it into a huge post and publish on your site. You’ll get Tweets and links from those influences (example)
• Try guest blogging (especially useful for new sites that have no contacts to start with). Use MyBlogGuest

Provide Something for Free:

• Work for free: Help non-profits and get their mentions;
• Set up annual rewards or regular contests and brand them to be recognized in your niche. Reward niche bloggers and influencers and get links from them
• Give away tools for free (plugins, apps, etc)

Reach out:

• Fix other people’s site: Find broken and redirected external links on high-authority resources and offer your content to link to instead.
• Research your competitors and learn their best tactics (while not making their mistakes);
• Create exhaustive research as to web site decision-makers in public and higher education libraries, then vary approaches as to SUPER personalized e-mails to see which approaches worked;
• Sign up to Google Alerts to be alerted of new link opportunities by email

Lastly:

The Best type of links come from people who have no idea about SEO but are passionate about their niche/content.. #ninja

Tools and Resources:

Please add your own working link building sources in the comments!